Prior to the invention described in U.S Pat. No. 4,923,720, the liquid spray application of coatings, such as paints, lacquers, enamels and varnishes, was effected solely through the use of organic solvents as viscosity reduction diluents. However, because of increased environmental concern, efforts have been directed to reducing the pollution resulting from painting and finishing operations. For this reason, there has been a great deal of emphasis placed on the development of new coatings technologies which diminish the emission of organic solvent vapors.
A number of technologies have emerged as having met most but not all of the performance and application requirements, and at the same time meeting emission requirements and regulations. They are: (a) powder coatings, (b) water-borne dispersions, (c) non-aqueous dispersions, and (d) high solids coatings. Each of these technologies has been employed in certain applications and each has found a niche in a particular industry. However, at the present time, none has provided the performance and application properties that were initially expected.
Powder coatings, for example, while providing ultra low emission of organic vapors, are generally characterized as having poor gloss or good gloss with heavy orange peel, poor distinctness of image gloss (DOI), and poor film uniformity. Moreover, to obtain even these limited performance properties generally requires excessive film thickness and/or high curing temperatures. Pigmentation of powder coatings is often difficult, requiring at times milling and extrusion of the polymer-pigment composite mixture followed by cryogenic grinding. In addition, changing colors of the coating often requires its complete cleaning, because of dust contamination of the application equipment and finishing area.
Water-borne coatings, on the other hand, are very difficult to apply under conditions of high relative humidity without serious coating defects. There defects result from the fact that under conditions of high humidity, water evaporates more slowly than the organic cosolvents of the coalescing aid, and as might be expected in the case of aqueous dispersions, the loss of the organic cosolvent/coalescing aid interferes with film formation. Poor gloss, poor uniformity, and pin holes unfortunately often result. Additionally, water-borne coatings are not as resistant to corrosive environments as are more conventional solvent-borne coatings.
Coatings applied with organic solvents at high solids levels avoid many of the pitfalls of powder and water-borne coatings. However, in these systems, the molecular weight of the polymer has been decreased and reactive functionality has been incorporated therein so that further polymerization and crosslinking can take place after the coating has been applied. It has been hoped that this type of coating will meet the ever-increasing regulatory requirements and yet meet the most exacting coatings performance demands. However, there is a limit as to the ability of this technology to meet the performance requirements of a commercial coating operation. Present high solids systems have difficulty in application to vertical surfaces without running and sagging of the coating. If they possess good reactivity, they often have poor shelf and pot life. However, if they have adequate shelf stability, they cure and/or crosslink slowly or require high temperature to effect an adequate coating on the substrate.
Clearly, what was needed was an environmentally safe, non-polluting diluent that can be used to thin very highly viscous polymer and coatings compositions to liquid spray application consistency. Such a diluent would allow utilization of the best aspects of organic solvent-borne coatings applications and performance while reducing the environmental concerns to an acceptable level. Such a coating system could meet the requirements of shop-applied and field-applied liquid spray coatings as well as factory-applied finishes and still be in compliance with environmental regulations.
Such a needed diluent was indeed found and is discussed in the aforementioned related patent which teaches, among other things, the utilization of supercritical fluids, such as supercritical carbon dioxide fluid, as diluents in highly viscous organic solvent-borne and/or highly viscous non-aqueous dispersions coatings compositions to dilute these compositions to application viscosity required for liquid spray techniques. In related U.S. patent application Ser. No. 631,680, it has been further discovered that the viscosity reduction effect may also be obtained with a subcritical compressed fluid, which fluid is a gas at standard conditions of 0.degree. C. and one atmosphere pressure.
As used herein, it will be understood that a "supercritical fluid" is a material which is at a temperature and pressure such that it is at, above, or slightly below its "critical point". As used herein, the "critical point" is the transition point at which the liquid and gaseous states of a substance merge into each other and represents the combination of the critical temperature and critical pressure for a given substance. The "critical temperature", as used herein, is defined as the temperature above which a gas cannot be liquified by an increase in pressure. The "critical pressure", as used herein, is defined as that pressure which is just sufficient to cause the appearance of two phases at the critical temperature.
Also as used herein, it will be understood that a "subcritical fluid" is a material which is at a temperature and/or pressure such that it is below its critical point. Such a subcritical fluid may be (i) below its critical temperature while being above its critical pressure, or (ii) below its critical pressure while being above its critical temperature, or (iii) below both its critical temperature and critical pressure.
As used herein, a "compressed fluid" is a supercritical fluid or, alternatively, a subcritical fluid which may be in its gaseous state, its liquid state, or a combination thereof depending upon the particular temperature and pressure to which it is subjected upon admixture with the composition which is to have its viscosity reduced and the vapor pressure of the fluid at that particular temperature, but which is in its gaseous state at standard conditions of 0.degree. C. and one atomposphere (STP).
Also as used herein, the phrases "coating composition" and "coating formulation" are understood to mean conventional coating formulations, that may or may not contain the polyester resins featured in the present invention, having no compressed fluid admixed therewith. The phrases "liquid mixture", "spray mixture", and "admixed coating composition" are meant to include an admixture of a coating composition or coating formulation with at least one compressed fluid.
With the objective of trying to diminish the emission of organic solvent vapors in mind so as to reduce pollution, it should be readily apparent that it would be highly desirable to replace as much of the organic solvent in a coating formulation as possible with the compressed fluids acting as viscosity reducing diluents. However, there is a limit as to the amount of compressed fluid that may be added to any given coating formulation. This limit is dicated by the miscibility characteristics of the compressed fluid with the coating formulation. If the compressed fluid is added much beyond such miscibility limit, a two phase separation results which may not be desirable for purposes of spraying and proper coating formation. Accordingly, it would be beneficial if a coating formulation, and particularly a polymer resin for such coating formulation, be available which has a higher miscibility with the compressed fluids.
So too, it would also be desirable to have a coating formulation, particularly a polymer resin, which posseses a relatively low viscosity. This would enable the utilization of a higher solids content within the coating formulation and a correspondingly lower amount of volatile organic solvents.
Clearly, a need exists to be able to accomplish these objectives. Preferably, these objectives are met while at the same time providing finished coatings which are equal to, if not better, than existing coatings with respect to the properties that they exhibit.
The present invention fills those needs by providing polyester compositions from which coating formulations may be made which have relatively low viscosities, have relatively high compatibility with compressed fluids, particularly compressed carbon dioxide, and which moreover provide coatings having an exceptional balance of properties, particularly high gloss, impact resistance, and water resistance.